库强对西南桦幼苗造林早期生长和养分利用的调控机制

Seedling quanlity is a crucial factor influencing early growth of seedlings after outplanting. Traditionally, growth indexes such as height and root collar diameter are used to evaluate seedling quality, regardless of that nutrient content has a positive effect on survival and early growth of outplanted seedlings.. Nutrient reserve can be established in seedlings through exponential fertilization and further improve seedling growth performance after outplanting. In this project, Betula aloides clone BY1 is selected for a case study, and nutrient reserves are constructed in its seedlings by modified exponential fertilization with three amounts of nitrogen (100、200、400 mgN seedling-1). Field trials are conducted to compare growth, nutrient uptake, carbon and nitrogen retranslocations of B. alnoides seedlings with different nutrient reserves under two treatments of fertilization using 13C and 15N labelling technique. Early growth, nutrient uptake, carbon and nitrogen contents as well as 13C and 15N abundances in old and new organs (root, stem, and leaf) are periodically investigated to reveal how nutrient reserve affects the early growth and nutrient utilization of B. alnoides after outplanting. The findings will provide theoretical evidences for its robusting seedling production and seedling quality evaluation.

苗木质量是影响造林早期生长的关键因素，传统苗木质量评价通常以苗高、地径等生长指标作为衡量标准，忽视了养分含量对造林成活率和早期生长的促进作用。通过指数施肥使苗木体内形成养分库，有利于提高造林效果。本项目以西南桦 (Betula alnoides) 无性系BY1组培苗为对象，设置3个施氮水平 (100、200、400 mgN 株-1)，利用修正指数施肥法构建苗木养分库，采用13C和15N同位素示踪技术，开展不同库强西南桦幼苗在施肥和不施肥条件下生长、养分吸收以及碳、氮运移和再分配研究，定期测定造林初期的幼苗生长和养分吸收以及根、茎、叶等新老器官的碳、氮含量、13C和15N丰度值等指标，揭示库强对西南桦造林早期生长和养分利用的影响，为其壮苗培育、苗木质量评价提供理论依据。

库强提高西南桦苗期生长和养分含量，但能否促进苗木移栽早期生长和养分吸收及其作用机制尚不清楚。本项目设置低、中、高3个苗期氮素处理（N100、N200和N400；每株苗木100、200和400mg N，以常规施氮量100mg株-1作为对照）构建不同水平养分库，随后将苗木分别移栽到两种肥力（F0和F10，每株0和10g缓释肥）的土壤中，分析西南桦幼苗根系形态及氮吸收动力学特征，探究苗期和移栽早期生长和氮素运移规律。研究结果表明：移栽前，与N100处理相比，N200处理的苗高、地径和生物量更大，而N400处理的氮浓度更高。移栽后，相对于N100处理，N200和N400处理的根长和根面积更优，而且N400处理的幼苗对铵态氮的吸收潜力和竞争力明显增强，可能由于重新转移到新根、新茎和新叶的15N均明显增加。此外，与F0处理相比，F10处理显著提高了移栽180天的根长、根尖数、苗高、地径和生物量，但降低了幼苗对铵态氮的吸收动力学参数，而不影响氮浓度和再转移。我们建议苗期施入中高剂量氮肥（200-400mgN株-1）促进西南桦移栽早期根系发育和苗木生长，由于苗木体内养分储备在移栽后会迅速耗尽，因此在移栽时添加肥料也许能促进苗木生长，特别是养分贫瘠的立地条件。